Not the Sharpest Knife in the Drawer

What does the idiom “Not the Sharpest Knife in the Drawer” mean?


Not very intelligent

For example:

“You have to explain things to Valerie several times, in different ways. I like her, but she’s not the sharpest knife in the drawer.”


There are many variants of this, all of the form “not the X-est Y in the Z.